The present invention relates to the diagnostic imaging arts. It particularly relates to computed tomography imaging employing a two-dimensional detector array that enables rapid acquisition of volumetric imaging data, and will be described with particular reference thereto. However, the invention will also find application in other types of detector arrays for a variety of imaging applications employing x-rays, visible light, or other types of radiation.
Computed tomography (CT) imaging typically employs an x-ray source that generates a fan-beam or cone-beam of x-rays that traverse an examination region. A subject arranged in the examination region interacts with and absorbs a portion of the traversing x-rays. A CT data acquisition system (DAS) including a two-dimensional detector array is arranged opposite the x-ray source to detect and measure intensities of the transmitted x-rays. Typically, the x-ray source and the DAS are mounted at opposite sides of a rotating gantry such that the gantry is rotated to obtain an angular range of projection views of the subject.
In helical CT imaging, the patient is advanced linearly through the examination region along a direction that is perpendicular to the gantry rotation plane to effectuate a helical orbiting of the x-ray source about the subject. X-ray absorption data obtained during the helical orbiting is reconstructed using filtered backprojection or another reconstruction method to generate a three-dimensional image representation of the subject or of a selected portion thereof.
The two-dimensional detector array of the DAS typically includes a scintillator crystal or array of scintillators which produce bursts of light, called scintillation events, responsive to impingement of x-rays onto the scintillator. A two-dimensional array of photodetectors such as photodiodes or photomultiplier tubes are arranged to view the scintillator and produce analog electrical signals corresponding to the scintillation events.
A problem arises in conventional CT imaging because the data is under-sampled. The spacing between samples typically corresponds to the detector spacing. The maximum continuous frequency response is fco=1/W where W is the detector spacing. However, Nyquist sampling theory calls for a sampling rate fsampling=2/W to avoid aliasing and other sampling-related artifacts. The undersampling can produce image artifacts and reduced resolution in the under-sampled direction.
To counteract the under-sampling, it is known to employ an x-ray source incorporating a dynamic focal spot, in which the focal spot is alternated or wobbled between two selected positions in the rotational or X-direction between DAS measurements to interleave samples. This effectively doubles the sampling rate in the X-direction to satisfy the Nyquist criterion. However, source wobbling is difficult to apply in the axial or Z-direction.
It is also known to employ a quarter ray offset in which rays from opposing 180° projection views are interleaved to effectively double the sampling rate in the rotational X-direction. Again, this method is generally inapplicable for the axial direction.
The under-sampling can also be counteracted by increasing the sampling rate, or by reducing the detector element size and increasing the number of signal channels. However, these approaches increase the cost, complexity, and bandwidth of the data acquisition system and associated signal processing electronics. These methods also reduce the signal to noise ratio by reducing the acquisition time or the detector area, respectively.
The present invention contemplates an improved apparatus and method that overcomes the aforementioned limitations and others.